In the customary way of producing flanges surrounding openings in deformable metal sheets for the purpose of strengthening the opening or preparing it to receive a tube, as in header assembles used for such applications as heat exchangers, it is customary first to pierce or perforate the sheet and then displace the portions of the sheet surrounding these holes from the plane of the sheet to form the flange. Such procedure is exemplified in prior U.S. Pat. Nos. 3,425,465 and 4,150,556. It has been found that when the hole is formed as by cutting out sections of the sheet or header plate and then deforming the sheet around the hole to form the flange, the edges of the sheet at the flange edge frequently split owing to the circumferential tensile deformation, so that it is not only difficult to form a joint with another piece of metal such as a tube soldered, welded or the like in the hole but, even when the joints are made, the splits are a major source of leakage. Furthermore, flange walls formed in this manner are of limited height, are often not parallel and their height tends to be uneven. In addition, their wall thickness gradually diminishes towards the edges. These features not only create further difficulties in tube-and-header assemblies but weaken the structure for other purposes as well.
Several methods have been suggested to overcome these difficulties. Thus, more material may be made available by drawing in material adjacent to the site of a flange by first creating a dimple, sometimes by reverse dimpling as in prior U.S. Pat. Nos. 1,699,361 and 3,412,593. The thickness of the flange may be made uniform by upsetting the formed flange in a separate operation, as in prior U.S. Pat. No. 2,859,510. More material may be made available for a thicker flange by compressing the sheet between two punches of equal size with a cross-sectional area equal to the inner dimensions of the future hole, as in prior U.S. Pat. No. 2,909,281.
Cracking of the flange edge can be delayed or prevented by a number of means. Removing the burr produced in punching out the hole is well known to increase the allowable diameter expansion in flanging. Further improvements can be achieved by extruding the flange after the hole has been deburred as in prior U.S. Pat. No. 3,412,593. Yet another solution is described by M. H. Williams in SAE paper No. 780,393 as a process sequence in which the hole is first pierced as in traditional flanging and then the flange formed by drawing between a punch and a back-up tool which maintains a compressive stress on the flange edge. This delays splitting and allows much deeper flanges to be formed. By the nature of the flanging process, the wall thickness of the flange still diminishes towards its edge. A parallel wall of uniform thickness can then be obtained, if so desired, in a subsequent ironing step. A total of 3 steps are thus required, and deformations attainable in the second and third steps are limited by both material and process limitations.